Grapevine Red Blotch Disease: An Emerging and Widespread Menace

As vintners in California recover from a harried, compact and early 2015 harvest, and concern themselves with
the vestiges of a continuing drought, there is apprehension about a new threat. An aggressive virus producing
Grapevine Red Blotch Disease (GRBV) is now pervasive in California vineyards of the North Coast, Central
Coast and San Joaquin Valley.

Reliable field data is lacking, but Cornell researchers reported in 2014 that the virus was widely distributed in
vineyards not only in California, but also throughout the United States including the states of Maryland, New
York, New Jersey, Oregon, Pennsylvania, Virginia and Washington (*1). Others have reported GRBV in Arizona,
Arkansas, Georgia, Idaho, North Carolina and Texas, as well as Canada.

GRBV is indiscriminate, infecting many wine grape cultivars including red-berried cultivars Cabernet
Sauvignon, Cabernet Franc, Malbec, Merlot, Mourvèdre, Petit Sirah and Petit Verdot, Pinot Noir and white-berried
cultivars Chardonnay, Pinot Gris, Riesling, Rousanne, Semillon and Viognier. Infected vines have been found in
first leaf as well as mature vineyards. The virus has also been found in table grapes.

Agri-Analysis in West Sacramento, California, reported in 2015 the percentage of positive samples in all
samples tested over the past three growing seasons (2013-2015). Results from a large number of grapevines
from various sources including existing vineyards and new planting material revealed the infection rate of Red
Blotch Disease to be between 15% and 25%.

Bob Martin, a research plant pathologist for the USDA-ARS Horticulture Crops Research Unit in Corvallis,
Oregon, told me, “Red Blotch occurs in Oregon for sure, based on what I have seen and testing we have done.
It has been discovered in about 15 to 20 vineyards by early 2014, with only a few vines affected in some
vineyards while the signs of infection are pervasive in others. It is moving very slowly in the Willamette Valley
and is spreading in southern Oregon, but I am not sure how fast.”

The Red Blotch virus is one of some 65 different viruses that have been identified in grapevines (Vitis and
Muscadinia) since 1962, but it is different from most other known grapevine infecting viruses in that its genetic
material is DNA, rather than RNA. It is a member of a new genus in the geminivirus family, Geminiviridae, that
carries the name GRBaV, or “Grapevine Red Blotch-Associated Virus,” to describe its association with the
grapevine and red blotch disease.

Jim Wolpert, a University of California at Davis-based Cooperative Extension viticulturist and Mike Anderson, a
University of California at Davis viticulture researcher, first discovered the disease in 2008 at the University of
California at Davis Oakville Station Experimental Vineyard in Napa Valley. Cabernet Sauvignon vines grown on
101-14 rootstock planted there in 2001 showed leaf changes (red blotches along leaf margins and red
foliar veins) that resembled those caused by leafroll virus, but the vines tested negative for that virus. The block
was found to have GRBaV in 46% of the vines.

The GRBaV virus has been present in California vines before this discovery. M. Al Rwahnih, A. Rowhani and D.
Golino from the Department of Plant Pathology at University of California at Davis analyzed specimens
originally collected by Professor Harold Olmo of the Department of Enology and Viticulture at University of
California at Davis between 1937 and 1950 (*2). One sample tested positive for GRBaV as Vitis vinifera cv. (early
Burgundy known as Abouriou) collected in Sonoma County in 1940. The leaves showed typical red blotch disease symptoms. This study suggests that GRBaV was present in California vineyards (at least where the
specimens were taken) 74 years ago.

The exact origin of GRBaV is undetermined. The virus has not been discovered outside North America
suggesting it may have originated here. The National Clonal Germplasm Repository (NCGR) in California has
found grapevine accessions from countries outside North American including Europe that have tested positive
for GRBaV, but this is not proof that the virus occurs in those countries.

According to Dr. Judit Monis, Plant Health Services Division Manager of Eurofins STA Laboratories in Gilroy,
California, and others, a lack of education, the confusion with Grapevine leafroll-associated virus (GLRaV),
nutritional deficiency, late season dormancy, insect feeding damage, environmental stresses, soil conditions,
and fungal pathogens have delayed the identification of GRBV in domestic grapevines. The possible
emergence of a more virulent strain of GRBaV and the failure to institute practices to curtail it, have led to the
spread of GRBV.

Researchers indicate that the symptoms of Red Blotch Disease in red-berried grapevine cultivars first appear
in June to July in the Northern Hemisphere as irregular blotches on older leaf blades on the basal portions of
shoots (*3). The blotches can expand and coalesce across most of the leaf blade and progressively move toward
the top of the canopy late in the season (August to October in the Northern Hemisphere). Affected leaves may
drop off prematurely. There are variations in reddening depending on the cultivar and rootstock.

The changes resemble leafroll disease, but the leaf margins in GRBV do not roll downward, and the leaf veins
in GRBV turn partly or fully red while leafroll has green veins. The visual diagnosis of GRBaV-infected vines
can be difficult, especially since grapevines often have multiple viruses at the same time, including leafroll
virus, that produce other symptoms, making DNA-based assays the only reliable method of accurate diagnosis.
Confounding the diagnosis is the determination that vines can test positive for the virus but have no symptoms
or symptoms may appear in one vintage and not another.

The two photos below show red blotch in a Russian River Valley vineyard taken after harvest in October 2015.
The vines tested positive for GRBaV.

The two photos below show red blotch signs in Pinot Noir leaves. They are from a report titled, “Red Blotch-
Research Status,” by Deborah Fravel of the USDA Agricultural Research Service (*4).

The photo below, also from Fravel’s report, shows the difference in foliar appearance between red blotch and
leafroll disease in Cabernet Franc vines.

In white-berried cultivars, visual diagnosis is even more challenging. The leaves show irregular, yellow chlorotic
areas that can become necrotic later in the growing season. In some cultivars, such as Sauvignon Blanc, the
disease may remain asymptomatic. The photo below, also from Fravel’s report, shows Grapevine red blotch
virus-associated disease in a Riesling vine.

The GRBaV virus was first reported in October 2012 at the 17th Congress of the International Council for the
Study of Virus and Virus-Like Disease of the Grapevine (ICVG) held at Davis, California, by researchers at
Cornell University and University of California at Davis, who had been working independently and
concurrently (*5).

Dr. Mysore Sudarshana, a plant pathologist with the USDA Agricultural Research Service at University of
California at Davis studied red blotch in vineyards in Napa Valley and worked with Maher Al Rwahnih, a
researcher at University of California at Davis Foundation Plant Services (FPS) to test the vines. Using
reverse-transcription polymerase chain reaction (PCR) tests, the virus was identified in Cabernet Franc,
Cabernet Sauvignon and Zinfandel grapevines that exhibited foliar symptoms of red blotches and red veins
accompanied by reduced sugar accumulation in the grapes at harvest. The researchers published the first
report of a DNA virus associated with red blotch disease in grapevines in California in 2013 (*6).

Cornell researchers found a declining Cabernet Franc vineyard in New York that showed symptoms suggestive
of leafroll, but the vines tested negative for all leafroll-associated viruses. The genome of this virus, tentatively
named “Grapevine cabernet franc-associated virus” or GCFaV, proved to be identical to the genome
discovered by Sudarshana and Rwahnih (*7). It was agreed at the 17th Congress (ICVG) to refer to the virus as
“Grapevine red blotch-associated virus” or GRBaV.

The relationship between the GRBaV virus and red blotch disease was at first met with resistance absent the
completion of Koch’s postulates. This year, Marc Fuchs and colleagues from the School of Integrative Plant
Science at Cornell University reported agroinoculation experiments at the 18th ICVG held September 7-11 in
Ankara, Turkey, that confirmed the hypothesis that GRBaV was the causal agent of red blotch disease. Fuchs
also confirmed that GRBaV was removable through microshoot-tip tissue culture.

A DNA-based polymerase chain reaction (PCR) assay was developed by Eurofins STA Laboratories soon after
GRBaV was reported at the ICVG in 2012. Eurofins HealthCheck Panel RB can detect GRBaV in any tissue of
the vine including leaf blades, petioles, clusters, green or dormant canes, cordons and trunks at any time of the season.
The virus is discoverable before the onset of symptoms in the affected grapevine. There are two genotypical
variants of red blotch.

The transmission methods of red blotch disease are not clearly defined although it is certain that grafting and
propagation with infected scion and rootstock can spread GRBaV. Growers have been frustrated because they
have obtained certified plant material for planting only to later find that GRBaV shows up in their new plantings,
since the PCR-based test for GRBaV only became available in October 2012.

The existence of a vector has been suggested by an increased incidence of red blotch disease over time in
young, healthy vineyards that are adjacent old, infected vineyards, short-distance spread of the virus,
clustering of infected vines in some vineyards, and the increase in diseased vines over time (*3). A possible
vector, the Virginia creeper leafhopper, has been identified in greenhouse experiments but not in the field.
Geminiviridae viruses are known to be transmittable by whiteflies, leafhoppers and treehoppers but every field
test over the past three years has failed to find evidence of an insect transmission. University of California at
Berkeley entomologist Dr. Kent Daane does not believe there is any conclusive evidence yet that suggests that
any of these vectors should be treated with insecticides because of red blotch.

There are conflicting reports on spread in vineyards by pruning and harvesting, and some growers have
already resorted to sterilizing pruning shears with bleach between each cut as well as sanitizing other
mechanical equipment. If such a step in pruning, hedging and harvesting proves to be necessary, the cost of
farming grapevines could easily double. Michael Anderson of the Department of Viticulture and Enology at
University of California Davis spoke to spread of the virus, saying, “The most important issue concerning red
blotch is understanding the mechanism(s) of vine-to-vine spread. It is one thing to do everything you can to
ensure clean planting material, but it is quite another to keep it that way.”

Bob Martin, a research plant pathologist in Oregon remarked to me about spread in that state. “The virus
seems to be clearly spreading in some vineyards. When symptomatic vines are removed, new plants exhibit
signs of the disease the following year. There are differing rates of spread depending on whom you talk to, but
very little work has been done with tagging vines and looking year after year to monitor spread. We know it’s a
problem but we don’t know how severe it is, and we don’t know how to control it.”

The effects of GRBD are much more devastating than leafroll. Sugar levels in grapes grown on vines affected
by red blotch can suffer a Brix reduction of up to 6º, much more than is seen with leafroll-diseased grapevines.
In addition, it has been found that in wines from diseased vines, ripening is delayed, berry weight is reduced,
yields are diminished, pH can be reduced, titratable acidity elevated, color reduced in red grapes and wines,
and phenolics and tannins disturbed in comparison to wines from non-diseased vines. In essence, once a
grapevine is infected and symptoms appear, the grapevine may not produce commercially useful grapes.

At the Sonoma County Grape Day event on February 18, 2015, held in Santa Rosa, California, Anita
Oberholster of the Department of Viticulture and Enology at University of California at Davis discussed how
GRBaV can limit grape development and affect wine made with grapes from virus-positive vines (*9). In extreme
cases of red blotch disease, red-berried vines produce pink grapes that never exceed 19º Brix. She reported
that in 2014, researches harvested Chardonnay, Merlot and Cabernet Sauvignon grapes from vines that tested
positive for GRBaV and from vines that did not. Most of the grapes from vines with red blotch had lower Brix
levels ranging from 4% to 20% less, with one Chardonnay sample showing no difference in Brix levels from a
virus-positive vine compared to one without the virus. Early on, the bottled wines made with grapes from red
blotch-positive vines showed reduced fruit character and more “artificial fruit” flavors. She said, “The wines had
a marked difference in taste with virus-positive wines displaying a thinner mouthfeel marked by green and
‘grainier’ tannins.”

There are conflicting reports on the effect of GRBaV on the longevity of vines, including some instances of
accelerated vine death, but the threat of disease spread within a vineyard and the resultant effects make this
nearly irrelevant.

There is currently no cure for GRBD and no proven vine management options that can restore normal fruit
quality in infected grapevines. Dr. Marc Fuchs of Cornell University recommends that symptomatic vines be
tagged in the fall and removed during the dormant season when insect activity is very low or absent. If greater
than 25% of a vineyard is diseased, the entire vineyard should be replaced. Some 4 to 10-year-old vineyards in
the North Coast have already been removed following recent harvests.

It is interesting to speculate that the 3-year drought in California and other climate changes have contributed to
the emergence of a more aggressive or at least opportunistic GRBaV virus. The virus appears to hibernate and
emerge in cool temperatures and activity may be made active by stress such as drought. Some have
suggested that global climate change has driven the virus to migrate from tropical climates to new areas.

We know that grapevines as well as all plants have disease resistance mechanisms, some of which are also
common in animal and human immune systems, that allow the plant to tolerate certain viruses within and show
little disease damage from those pathogens. The interaction of the plant, a pathogen and the environmental
conditions is known as the disease triangle. A significant change in the environment, such as drought, can tip
the scale in favor of a pathogen and disease damage may appear.

Noted winegrower and winemaker Ted Lemon commented to me about disease resistance in grapevines.
“There is no doubt that viruses can remain dormant in grapevines for many years. Like humans, I would expect
that stress, including drought, could favor expression of the virus. As for the mechanism(s) that induce
expression, that is a million dollar question.”

Ted elaborated further on virus detection and management in grapevines. “From a viticultural point of view,
most of our leading researchers and government agencies take the approach of somehow trying to ‘guarantee’
that plant material will be virus free. They have been singing this siren song for decades. Our plant material has
never been virus free. It is clearer than ever that most of the nursery sources and FPS vineyards are full of
virus.”

Ted advocates more ecological research and less emphasis on cutting-edge virology research. He concludes,
“Our goal for people and plants ought not to try to make them virus and disease free, but to create vibrant,
healthy ecosystems (be they on the farm or in the home) where balance prevents disease associated with virus
expression. In the case of humans, the moral question pushes us to try to insure that no one suffers from virus-induced
disease and we err on the side of too much medicine. In the plant world, we can more easily accept
the risk that a portion of plants fail if this creates better health in the overall population.”

Ted’s words are proven out by a report (Ref National Clean) that indicates GRBaV is widely prevalent in
California Department of Food & Agriculture (CDFA)-certified grapevine increase blocks, especially scion
blocks. It is also found in CDFA-certified rootstock blocks but at lower frequency than in scion materials.
GRBaV is frequently found in non-CDFA-certified scion field selections and clones (*10).

The health of foundation stocks and nursery stocks is critical to insure clean vines are planted in the future. The
new Grapevine Foundation Block at Russell Ranch at UC Davis FPS was tested in 2013 and is completely free
of GRBaV. This vineyard will become the primary source of wood for certified nurseries in the future. A low
incidence of GRBaV (less than 1%) has been discovered at the FPS Classic Foundation Vineyard.

Nurseries are investing in establishing new increase blocks distant from existing blocks and other vineyards to
hopefully provide clean rootstock and scion cuttings for future vineyard plantings.

Current vineyard management options for GRBV include the following (*10).

(1) Only plant healthy certified grapevine material that has been screened for viruses and certified free of
GRBaV by micro-shoot tip culture, either from Russell Ranch at FPS in California or Clean Plant Center
Northwest (Washington State University, Prosser, WA).

(2) Growers should consider independent lab testing of all orders from nurseries before accepting delivery
since the CDFA nursery certification program currently has inadequate protocols for dealing with red blotch
disease. When growers who buy CDFA-certified stock, they cannot assume they are buying clean materials.

(3) Tag affected vines to allow for future management as more information is available. Remove individual
contaminated vines when symptoms become apparent.

(4) Do not propagate from any vines, even asymptomatic ones, in an existing vineyard unless individual vines
have been tested for known viruses.

California winegrape growers with losses due to GRBaV are now eligible for financial assistance for vine
replacement from the U.S. Department of Agriculture’s (USDA) Tree Assistance Program (TAP). The program
will be in place at least until 2018. There are 30 local USDA Farm Service Agency (FSA) offices in California
where USDA Form AD-1026 may be filed to receive TAP funds. For complete details, visit www.cawg.org/
docuserfiles/files/July%202015%20Crush.pdf.

No similar program is currently in effect in Oregon, but in late 2014, a group of growers, nursery operators,
Oregon Watershed Restoration Inventory (OWRI) faculty, and Oregon Department of Agriculture (ODA) plant
health scientists convened to share information, provide an over view of the grapevine virus situation in Oregon
vineyards, and plan research into management. ODA plant pathologists together with the Oregon Wine Board
planned to apply for an ODA specialty crop block grant to do a survey of red blotch in Oregon vineyards. The
group will meet again in December 2015 (*11). 95% of Oregon grapevine nursery stock comes from California, so
potentially infected plants may have arrived before testing for GRBaV.

(*5) Al Rwahnih, M., et al., “Virus in grapevines affected by red blotch disease in California,” in Proceedings of the
17th Congress of the International Council of the Study of Virus and Virus-like Disease of the Grapevine
(ICVG), Davis, California, October 2012: www.ucanr.edu/sites/ICVG/Documents_to_share/.